In conventional automotive fuel hoses, usually nitrile rubber (acrylonitrile-butadiene rubber) is utilized as a fundamental material of an innermost layer of the hoses and chloroprene rubber as a covering material.
A trend in recent automobiles is considerable complication of their fuel systems due to the employment of various measures to achieve satisfactory control of exhaust emission. In many recent automobiles, therefore, gasoline flowing through the fuel line is liable to undergo a considerable rise in its temperature by the influence of exhaust emission control means. Then, there is a strong possibility that the heated gasoline is partially oxidized to form so-called sour gasoline which is highly erosive to many organic materials including synthetic rubbers. In automobiles where the fuel system is of the fuel injection type, an erosive action of sour gasoline on the material of fuel hose is further promoted by the pressure applied to the fuel. When such changes in the condition of the fuel is taken into consideration, nitrile rubber as the fuel hose material is no longer satisfactory in its heat resistance and oxidation resistance. In other words, there arises a problem that automotive fuel hoses which employ nitrile rubber as the fundamental material cannot be expected to have a sufficient service life. Since deterioration of the fuel hose is liable to result in fuel leak and hence constitutes a serious danger, now it is necessary to utilize a fuel hose having a sufficient resistance to sour gasoline.
Under such a situation, attention has been given to relatively recently developed synthetic rubbers, particularly to fluorine rubber (fluorine-containing hydrocarbon elastomer) and epichlorohydrin rubber, as fuel hose materials superior to nitrile rubber in heat- and solvent-resistance. However, practical applications of these new candidates to commercial fuel hoses have encountered problems because of each of these new rubbers has not only merits but also certain demerits.
Fluorine rubber is excellent in heat resistance and solvent resistance and has been regarded as sufficiently resistant even to sour gasoline. However, fluorine rubber is too costly to use as a material for a component of mass-produced automobiles. For example, it costs about twenty times more than nitrile rubber conventionally used for automotive fuel hoses. Moreover, fluorine rubber is not fully satisfactory in its resistance to cold. The practicability of fluorine rubber currently available as a hose material at low temperatures is limited to the level of about -25.degree. C. or about -30.degree. C. at the best, while automotive fuel hoses must be expected to experience not only considerably high temperatures, even above 100.degree. C., in engine rooms but also very low temperatures such as -40.degree. C. or below during operation in cold terrains. There is a possibility to obtain a special type of fluorine rubber whose cold resistance extends to -40.degree. C. or below, but the price of such rubber will be five to ten times as high as that of currently marketed ordinary fluorine rubber.
Epichlorohydrin rubber is relatively low in price and excellent in cold resistance but is rather poor in its resistance to oxidized fuel, particularly, in a heated state. When this rubber is long kept in contact with a hot and oxidized gasoline, it tends to soften and lose its resiliency, and this tendency is promoted where the fuel is pressurized. Therefore, it will be impermissible to use epichlorohydrin rubber as the material for an innermost layer of an automotive fuel hose.
Some synthetic resins other than synthetic rubbers have also been used for conveyance of organic liquids. As an example of tubular laminates utilizing a synthetic resin, Japanese Utility Model Application Publication No. 49(1974)-7938 shows a "hose" comprising a tube of a synthetic resin such as nylon or Teflon as the innermost layer of a relatively small thickness and a tubular rubber layer of a larger thickness covering the outside of the resin tube. (No description is given about the type of the rubber for the covering layer.) Certainly this type of hose will be excellent in resistance to heat and solvent. However, this hose cannot have a sufficient flexibility even though the inner tube has a considerably small wall-thickness since neither nylon nor Teflon exhibits rubbery or elastomeric behavior. It is not proposed to use this hose as an automotive fuel hose. If it is intended to construct an automotive fuel line by using this hose, it will be necessary to employ specially designed and probably complicated types of clamps and bandage tubes for the purpose of surely precluding the occurrence of fuel leak during long operation of the fuel system, because this hose is inferior to true rubber hoses in rubbery resiliency, flexibility, bendability and the extent of elongation within the limit of elasticity. Even when such a measure is taken, there will remain a fear that mechanical vibrations imposed on the fuel line will cause distortion or loosening of couplings of the hose with metal conduits and resultant leakage of fuel. Besides, this hose will suffer an insufficient durability due to a separation tendency of its rubber covering layer from the synthetic resin tube. Therefore, this type of hose is unsuitable for automotive uses.